PURPOSE: To prevent the entry of a noise component and operate a computer system stably by grounding the system through a low-resistance noise filter which removes a high frequency component.

CONSTITUTION: The electric power from an AC power source 1 as a commercial power source is supplied to an AC/DC converter 3 after being passed through the low-pass filter 2 to remove an external noise. The direct current outputed from it is supplied to a circuit group which operates in a high frequency band such as a printed board surrounded with a grounding surface 4 and those are covered with a housing 6 which has metal shield effect. In this case, the grounding surface 4 is grounded to a ground electrode 9 through the low-resistance noise filter 11 which stops the high frequency component. The housing 6 is grounded directly to the ground 9 through a grounding line.